What Blood Can Tell You

Blood tests are a part of veterinary medicine known as laboratory medicine. This branch of medicine is used by your veterinarian—along with physical examination findings—to aid, confirm, or disprove a suspected diagnosis.

Photo: Erica Larson, News Editor

Your veterinarian wants to do some blood tests on your horse. It's a common term to anyone who has been around horses very long, but it is also a very non-specific term. What kind of tests, and what will the results tell him (and you)? Before you can understand what blood tests are and how they can help diagnose ailments in your horse, it's a good idea to understand some of the basics about the study of blood—hematology.

Blood tests are a part of veterinary medicine known as laboratory medicine. This branch of medicine is used by your veterinarian—along with physical examination findings—to aid, confirm, or disprove a suspected diagnosis. Laboratory medicine includes a wide range of sub-specialties including hematology (the study of red and white blood cells), clinical chemistry (which helps evaluate organ dysfunction), and cytology (the study of cells, often referred to as clinical pathology). For this article, we will only discuss hematology. The tests will not usually give an exact diagnosis, but can at least determine if an organ system (i.e., liver or kidney) is not functioning normally or how the body is responding to an infection.

Blood is approximately 60% (+/-10%) water, 39% (+/-10%) red corpuscles or red blood cells (RBC), and the remainder (about 1%) is comprised of hundreds of elements, electrolytes, enzymes, and proteins. The RBC is a single cell with an exterior membrane housing hemoglobin (protein-iron complex), water, and electrolytes. The inside of the RBC has a relatively high concentration of potassium and a low concentration of sodium and chloride compared to the liquid portion of the blood—the plasma—which has a relatively high concentration of sodium and chloride and low concentration of potassium.

The blood of mammals is red mostly because iron-containing hemoglobin absorbs the red spectrum of light, but this can be different for other species. For example, the blood corpuscle of the "limulus" crab (the blue crab) has cobalt instead of iron within its "heme" complex, and as a result has blood that is blue in color.

There's about one white blood cell (WBC) for every 100 red blood cells. White blood cells consist of neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The white blood cells will be discussed in more detail later in this article.

When most people think of the bloodstream, the first thing that comes to mind is the transport of oxygen from the lungs to the rest of the body. While this is a very important function, it is actually only one of many. The bloodstream is the interstate highway system of the body, allowing for many things to get around as needed, such as white blood cells to the site of an infection, clotting proteins and repair cells to the site of damaged tissue, proteins of the immune system to where they are needed, and nutrients to be distributed from the gastrointestinal tract to the rest of the body.

The mechanism by which the horse's body maintains itself in a stable (and consistent) nature is called "homeostasis." When all is normal, most of the components of blood are regulated to a very narrow concentration that varies only slightly from horse to horse. Because of that fact, a reference range for what is normal is available for comparison. Many blood components change in a very predictable way relative to a disease process; therefore, blood tests can be extremely valuable aids in disease diagnosis and determining health status.

Blood Components

When discussing blood testing, concentration and math enters the discussion. The first basic evaluation is the percentage of solids to fluid. The number of red and white blood cells is typically reported as how many cells per volume of blood, and the other components of blood being reported in weight quantity per volume of blood. The metric system is standard, using deciliter (1/10 of a liter), milliliter (1/1,000), and microliter (1/1,000,000), respectively. The solids are measured in weight using gram, milligram (1/1,000 of a gram), and microgram (1/1,000,000) of a gram, respectively. Some of the blood components are enzymes. These are measured by their level of activity per volume of blood and are typically reported as "IU" (international units).

The RBC's main function is to carry oxygen throughout the body, while the various WBCs mainly carry out immune system functions. The red and white blood cells are manufactured within the bone marrow and released into the bloodstream. A small quantity of red and white blood cells are manufactured in the spleen. Platelets are cells that function in blood clotting, and are also manufactured in the bone marrow.

The main protein in the blood is albumin, which is manufactured in the liver, as are other proteins, including ones involved with blood clotting. One example is fibrinogen.

The main proteins of the immune system—the immunoglobulins (IgG, IgM, IgA, IgE)—are manufactured by lymphocytes, one type of white blood cell. Enzymes in the blood that are manufactured by—and related to—certain organ systems, and waste products of cellular metabolism related to certain organ systems, are routinely measured in blood testing that will be discussed later in this article.

Complete Blood Count (CBC)

Hematology is used to examine a horse's red and white blood cell counts. The common name for evaluating these components of the horse's blood is a complete blood count or CBC. The first test in this category is the packed cell volume or PCV (also called hematocrit). The PCV is the percentage of red blood cells compared to
the liquid component of blood, called the plasma. Blood is placed in a small glass tube and centrifuged (spun at a high speed to separate different components), thus separating the red cells and giving you a means of calculating the red cell mass. A low red cell mass would indicate anemia, and an increased red cell mass would indicate dehydration (or an overproduction of red cells, which is extremely rare in the horse).

The normal PCV for the horse is approximately 31-47% (the relatively wide range of normal is due to variation based on hydration status). While performing this test, it is also common to measure the total solids of the plasma by a light refraction technique. The total solids measurement looks at all of the blood proteins; if elevated, it is indicative of dehydration. A more detailed analysis of the blood proteins is necessary to evaluate other disease processes.

A CBC is a more complete evaluation of the cellular components. The RBC and WBC count can be performed by hand using a microscope and special counting chambers, but it is more commonly performed using automated hematology equipment. The RBC count is reported in millions of cells per microliter of blood. The normal equine range is 6-12 million red blood cells per microliter of blood. There are many causes of anemia (low red blood cell count), including chronic parasitism, blood loss, and chronic inflammation.

This reminds us that a single laboratory test only helps a little with finding the cause of a problem, but when used in combination with other tests and clinical examination, the puzzle can become more clear.

For example, with chronic parasitism there is often weight loss and a low blood protein concentration, and with chronic inflammation there might be low-grade fevers and an increased blood protein concentration. Hemoglobin (the protein within the RBC that contains iron) is normally 10-18 grams per deciliter of blood; a low hemoglobin level in conjunction with anemia would be an indication of iron deficiency anemia. The more common cause of iron deficiency anemia is not dietary, but chronic inflammation or the "anemia of chronic disease" (iron is tied up in the infection-fighting processes of the immune system).

There normally are 100,000-250,000 platelets per microliter of blood; a low blood platelet level is typically an indication of use by platelet blood clotting or a lack of production by the bone marrow. There normally are 5,500-12,000 WBCs per microliter of blood.

The different types of WBCs are counted on the "differential" (the part of the test that counts the different types of cells) and are evaluated as a percentage of the total. There are the segmented or mature neutrophils (normally about 60% of the total), the band or immature neutrophils (normally 0%, more on this later), lymphocytes (normally about 40%), monocytes (normally 0-1%), eosinophils (normally 0-1%), and basophils (normally 0-1%).

Looking at the total WBC count and the differential can help diagnose various types of inflammation. For example, with severe acute inflammation/infection, the WBC count might decrease with the appearance of immature neutrophils. With more slowly developing or chronic inflammation/infection, the total WBC count tends to increase. With allergic-type inflammation, increases in the numbers of eosinophils and basophils might be noted. The monocyte is the "clean-up" cell of the body, so with chronic inflammation there usually is an increase in the number of monocytes.

In addition to counting the number of WBCs, lab technicians typically evaluate cells under the microscope for specific changes in cellular structure (morphology) that indicate inflammation/infection. Such changes are commonly called "toxic" changes of the neutrophils. With WBC counts, you often have more information after a few counts have been performed so you can determine a trend in the changes.

The WBC count is also useful in determining if you are getting a response to treatment. For example, with a mild/ moderate case of pneumonia, you would expect an increase in the WBC count. If the antibiotic of choice is correct, you would expect—in addition to improvement in the clinical signs—the WBC count to decrease toward the normal range.

The WBCs "patrol" throughout the blood stream until needed. For example, picture a group of bacteria trying to set up shop in lung tissue. Numerous factors trigger events that allow the WBCs to migrate out of the bloodstream and collect around the site of infection. From there the reactions are numerous and complex, but essentially the neutrophils attack and neutralize the invading bacteria. The lymphocytes mainly manufacture proteins that in addition to helping defend against direct attack of the invading bacteria will alert the rest of the immune system to the action going on within the lung. Also, the monocytes are converted to cells called macrophages that essentially digest and clean up the dead bacteria and damaged lung tissue (the repair process).

If the infection is extremely intense and involves a vigorous bacteria (or virus), the WBCs are consumed rapidly and the total count decreases because consumption exceeds production. With less intense and more chronic infections, the WBC count increases.

Take-Home Message

Blood testing can provide important information, but it almost always needs to be interpreted relative to the clinical examination findings, history, and additional testing in order to arrive at an accurate diagnosis of the problem.